1. Field of the Invention
The invention is related generally to the field of interpretation of measurements made by well logging instruments for the purpose of determining the properties of earth formations.
2. Description of the Related Art
Electrical, electromagnetic induction, and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, provide information regarding petrophysical properties of the formation and the fluids therein.
Due to the complexity and variation within some formations, accurate determinations of formation resistivity and other quantities may be difficult to obtain. For example, in the case of thinly laminated sand and shale sequences, it is difficult to accurately resolve the electrical properties of the individual beds or laminates and thus to ascertain information regarding the presence of hydrocarbon deposits.
That is, some of the thin beds or regions of thin laminates are below the resolution of logging tools and are usually interpreted as anisotropic formations. The measurements taken provide indications of bulk anisotropic resistivity, which are then further evaluated to provide indications of formation properties.
To interpret the anisotropy measurement for a sand and shale sequence, bulk resistivity is determined for both the transverse direction and the normal direction. Relations between the determined bulk resistivities and the predicted bulk resistivities are used to calculate the sand bed resistivity. Prediction of the bulk resistivities calls for knowledge of the resistivity of the shale component. Because shales are often anisotropic, knowledge of the shale anisotropy must be available in order to accurately relate the bulk resistivities to the sand resistivity and thus accurately calculate the sand resistivity from the measurement, which. will depend on the shale anisotropy. Determination of the shale anisotropy in a sand and shale sequence is difficult because the bed thickness is beyond the resolution capabilities of most logging tools. At present, borehole imaging methods seem to be the only choice for measuring microscopic formation (shale) anisotropy in situ.
One exemplary tool for such imaging is disclosed in U.S. Pat. No. 6,191,588, entitled “Methods and Apparatus for Imaging Earth Formations with a Current Source, a Current Drain, and a Matrix of Voltage Electrodes Therebetween,” issued Feb. 20, 2001 to Chen. One method for use of this apparatus is disclosed in U.S. Pat. No. 6,765,386, entitled “Galvanic Method of Measuring Electrical Anisotropy” issued Jul. 20, 2004 to Gianzero et al. The disclosures of both of these patents are incorporated herein by reference in their entirety.
Referring to U.S. Pat. No. 6,191,588, the borehole imaging apparatus includes a tool having an array of voltage electrode buttons mounted on a non-conductive pad. A current source and a current return are preferably located on the non-conductive pads at opposite ends thereof. The locations of the current source and return are designed to force a current to flow in the formation parallel to the pad face and non-parallel to the formation boundary layers. The voltage difference between a pair of buttons in the array is proportional to the resistivity of the formation bed adjacent to the buttons. The ratio of voltage differences between two nearby pairs of electrode buttons provides a quantitative measurement of the ratio of shallow resistivity. The resolution of the image produced by the tool is determined only by the spacing of the buttons.
However, when a trajectory of the borehole through which the apparatus travels is not perpendicular to a bedding plane, prior art techniques do not correctly characterize the formation resistivity.
What is needed is a method for measuring thin-bed resistivity anisotropy using an electrical or electromagnetic based well logging tool. Preferably, the method consistently provides accurate and reliable data without regard for the trajectory of the borehole and is computationally efficient.